Hello and welcome to JustAnswer!Most likely the trans is in limp in rather than taking off and staying in first. Depending on which trans you have limp in is either second or third gear.When the trans controller sees an issue it thinks can cause potential damage or thinks it doesn't have complete control it will shut off the relay that powers the solenoid pack and lock you in a single gear.There are many possible causes for limp in including the trans controller, trans relay, wiring, input and output speed sensors, the range sensor, solenoids and pressure switches, and internal trans issues. The most common cause of imp in would be the solenoid pack.The first step in diagnosing the problem will be to see what fault codes are seting in the TCM.